The rapid expansion of the number of cellular radiotelephones coupled with the desire to provide additional senrices has prompted the use of an improved transmission technique, called time division multiple access (TDMA). TDMA increases cellular system capacity over the current U.S. analog AMPS cellular system (Electronics Industries Associates Standard EIA-553) through the use of digital modulation and speech coding techniques. A TDMA signal transmission for the U.S. digital cellular system (Electronics Industries Associates Standard IS-54) is comprised of a continuous series of time slots. A radiotelephone operating in the U.S. digital cellular system only uses every third time slot.
A linear modulation technique, called .pi.4 differential quadrature phase shifted keying (.pi./4 DQPSK), is used to transmit the digital information over the cellular radio channel. The use of linear modulation in the U.S. digital cellular system provides spectral efficiency allowing the use of 48.6 kbps channel data rates. .pi./4 DQPSK transmits the data information by encoding consecutive pairs of bits, commonly known as symbols, into one of four phase angles (.+-..pi.4, .+-.3.pi./4) based upon gray encoding. These angles are then differentially encoded to produce an eight point constellation.
Transmitters designed for use in the U.S. digital cellular system are required to operate in both the TDMA digital mode and FM analog mode, referred to as dual mode operation. The TDMA digital mode uses the .pi./4 DQPSK modulation, and can be implemented using a linear transmitter. The FM analog mode uses conventional frequency modulation and allows the use of higher efficiency non-linear transmitters.
The linear transmitter is relatively inefficient in terms of average power out when operated non-linearly in the FM analog mode, but this is not as severe a problem in the TDMA digital mode since the linear transmitter is switched on with a one-third duty cycle. In other words, the linear transmitter is only on for one out of every three time slots in the TDMA digital mode. Since the linear transmitter is on only one third of the time, current drain in the linear transmitter may be actually less than that of continuously-operated non-linear transmitters in existing FM products.
There is a problem, however, when the linear transmitter is operated continuously in the FM analog mode at the same average power level as in the TDMA digital mode. The linear transmitter will have relatively poor efficiency and will draw much more current than conventional non-linear transmitters. As a result, there is a need for an efficient dual mode power amplifier network for use in both TDMA digital mode and FM analog mode.